1. Field of the Invention
The present invention relates to a prism sheet employed in a backlight unit, particularly a prism sheet having no structural deformation.
2. Description of the Related Art
A liquid crystal display (hereinafter, referred to as “LCD”) device is an electronic device changing and conveying various electric information generated from various apparatuses to visual information by using change of transmissivity of liquid crystals.
The LCD device is a device displaying information, but has no self-light emitting source. Thus, the LCD device needs a separate device in the backside in order to brighten the whole screen of the LCD device uniformly. The device providing a light is a backlight unit (hereinafter, referred to as “BLU”).
According to the installation method of cold cathode fluorescent lamp (hereinafter, referred to as “CCFL”), the BLU may be made in two methods: a direct-light method in which a lamp is located under an LCD panel, and an edge-light method in which a lamp is disposed on the side of a light guiding plate.
FIG. 1 is a cross-cross-sectional view illustrating a conventional BLU of a LCD device, and FIG. 2 is a cross-cross-sectional view illustrating the constitution of a prism sheet in FIG. 1.
In FIG. 1, the BLU is driven by the edge-light method, and includes a light source unit 110, a light guiding plate 120, a reflective sheet 130, and an optical film 140.
The light source unit 110 includes at least one CCFL 112 and a light source reflector 114.
The CCFL 112 generates a light having a predetermined wavelength.
The light source reflector 114 reflects the light generated from the CCFL 112 to the light guiding plate 120, and then the amount of incident light to the light guiding plate 120 is increased.
The light generated from the CCFL 112 is reflected by the light source reflector 114 and the reflective sheet 130. Then, the reflecting light is diffused uniformly through the light guiding plate 120.
The optical film 140 includes a diffuser 142, a prism sheet 144 and a protection sheet 146.
The light which is diffused uniformly in the light guiding plate 120 passes through the diffuser 142. The diffuser 142 makes the brightness uniformly, and widens the viewing angle of the LCD.
The brightness of light passed through the diffuser 142 is decreased rapidly. To prevent it, a prism sheet 144 is used.
In FIG. 2, the prism sheet 144 includes a base film 150, a prism base 152, and a plurality of prisms 154. Here, the prism base 152 and the prisms 154 are formed in sequence on the base film 150. The prism sheet 144 refracts the light that is transmitted from the diffuser 142, is incident to the base film 150, and passes through the prisms 154. Then, the incident light with low angle is condensed in the direction of the protection sheet 146, and the brightness is increased within effective viewing angle of the LCD.
The protection sheet 146 is disposed on the prism sheet 144, and prevents the prism sheet 144 from scratch, and widens the viewing angle of the LCD which is narrowed by the prism sheet 144.
In case BLU 100 continues to be used, the temperature of BLU 100 is increased by heat generated from CCFL 112.
Accordingly, the prism sheet 144 was deformed or rumpled by heat, and the function of the prism sheet 144 is deteriorated.
Also, the prism sheet 144 may be scratched by a slight impact.